Method of processing marine seismic data and a method of seismic surveying

ABSTRACT

A method of determining seismic properties of a layer of the seabed, in particular a surface or near-surface layer ( 5 ), comprises directing seismic energy propagating in a first mode at a boundary face of the layer so as to cause partial mode conversion of the seismic energy at the boundary face. For example partial mode conversion may occur when seismic energy propagates upwards through the interface between a surface or near-surface layer ( 5 ) and the basement ( 6 ), owing to the difference in seismic properties between the surface or near-surface layer ( 5 ) and the basement ( 6 ). In the invention, the two modes of seismic energy—that is the initial mode and the mode generated by mode conversions at the interface—are received at a receiver. The difference in travel time of the two modes between the interface and the receiver is determined from the seismic data acquired by the receiver.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of processing marine seismicdata, and in particular relates to a method of processing marine seismicdata that includes two modes of seismic energy propagation, one modearising from partial mode conversion at a boundary of a layer of theearth. The method of the invention provides an estimate of static shiftscaused by a layer at or near the earth's surface that has differentseismic properties from the underlying layers. The invention alsorelates to a method of seismic surveying that includes processingacquired seismic data in the above way.

2. Description of the Related Art

FIG. 1 is a schematic view of a marine, seabed seismic surveyingarrangement. In this surveying arrangement, seismic energy is emitted bya seismic source 1 that is suspended beneath the sea-surface from atowing vessel 2. The seismic energy is emitted downwards, passes intothe earth's interior and is reflected by a geological feature that actsas a reflector 3. The reflected seismic energy passes upwards throughthe earth's interior, into the sea and is detected by a seismic receiver4 disposed on the sea-bed. Information about the earth's interior isobtained, for instance, by determining the travel time of seismic energyfrom the seismic source 1 to the receiver 4. Only one reflector 3 isshown in FIG. 1 but, in practice, a number of geological features withinthe earth's interior will act as partial reflectors for the seismicenergy. Moreover, only one source 1 and one receiver 4 are shown in FIG.1 but in practice a marine seismic surveying arrangement may have anarray of sources and an array of receivers.

The geological structure of the earth is not uniform. One problem inprocessing marine seismic data is that frequently there is a layer 5 ator near the surface whose properties may well be significantly differentfrom the properties of the underlying geological structure 6 hereinafterreferred to as the “basement”). This can occur if, for example, there isa layer at or near the earth's surface that is less consolidated thanthe basement. In particular, the velocity of seismic energy may besignificantly lower in the surface or near-surface layer 5 than in thebasement 6, and such a surface or near-surface layer is thus generallyknown as a “low-velocity layer” (or LVL). This difference in velocitywill produce a shift in the travel time of seismic energy compared tothe travel time that would be recorded if the surface or near-surfacelayer and the basement had identical seismic properties, and theseshifts in travel time are generally known as “static shifts”, or just“statics”.

The low-velocity layer 5 is shown as a surface layer FIG. 1, but it neednot extend to the surface and there could be a further layer overlyingthe low-velocity layer.

The static shift generated by a surface or near-surface low-velocitylayer 5 depends on the thickness of the layer, and on the velocity ofpropagation of seismic energy through the layer. Lateral variationsusually occur in both the thickness of a low-velocity layer 5 and thepropagation velocity through the layer, so that the static shiftobserved at a seismic receiver at one location is likely to be differentfrom the static shift observed at a receiver at another location. To afirst approximation, the entire data set recorded at one receiver willbe advanced or delayed by a static time shift relative to data recordedat another receiver.

SUMMARY OF THE INVENTION

It is highly desirable to take account of the static shift whenprocessing seismic data. Unless these static shifts are removed from theseismic data, ambiguity will exist as to whether variations in arrivaltimes of seismic events from deeper layers are due to variations in thedepth or lateral locations of those deeper layers, or simply arise owingto propagation effects in the low-velocity layer 5.

The present invention provides a method of processing seismic dataincluding corresponding first and second modes of seismic energy, themethod comprising the step of processing the seismic data to obtain thetravel time difference through the layer between seismic energypropagating in the first mode and seismic energy propagating in thesecond mode.

In a preferred embodiment, the second mode was generated by partial modeconversion of the first mode at a boundary face of a layer of theseabed.

It will be seen from FIG. 1 that the seismic energy passes through thelow-velocity layer 5 twice, once as it travels from the seismic source 1to the reflector 3, and again as it travels from the reflector 3 to thereceiver 4. Both traverses of the low-velocity layer will cause staticshifts; the shift caused as the downwardly propagating seismic energypasses through the low-velocity layer 5 is known as the source-sidestatic shift, and the shift caused as the upwardly propagating energypasses through the low-velocity layer 5 is known as the receiver-sidestatic shift. The present invention provides a method of estimating thereceiver-side static shift.

The method of the invention may be used to process pre-existing seismicdata Alternatively, it may be incorporated in a method of seismicsurveying for processing the data as it is acquired or subsequently.

One embodiment of the present invention assumes that the contrast inphysical properties between the basement 6 and the low-velocity layer 5is sufficiently great that significant conversion between a p-mode ofenergy propagation and an s-mode of energy propagation, or vice-versa,takes place as seismic energy propagates upwards through the boundarybetween the basement 6 and the low-velocity layer 5. As a result of thismode conversion, for every p-mode event recorded by a seismic receiverabove the boundary between the low-velocity layer and the basement therewill be a corresponding s-mode event. The p-mode event and thecorresponding s-mode event will occur at different times, because thep-mode and s-mode propagation velocities in the low-velocity layer arenot equal to one-another. However, since the mode conversion occurs atthe lower boundary of the low-velocity layer, the time delay between thep-mode event and the corresponding s-mode event must arise as the resultof different velocities for the two modes in the low-velocity layer. Thetime delay between the p-mode event and the corresponding s-mode eventwill not be significantly influenced by the properties of the basement.The travel time difference between the p- and s-modes through thelow-velocity layer can thus be readily determined by identifying ap-mode event and the corresponding s-mode event in the seismic data anddetermining the time delay between the p-mode event and thecorresponding s-mode event.

Mode conversion may also occur when a downwardly propagating waveundergoes reflection at the interface between the low-velocity layer 5and the basement 6. The invention may be applied to this case since,where mode conversion occurs on reflection, the reflected signal willcontain a p-component and an s-component, and the two components willhave different travel times through the low velocity layer 5.

Mode conversion may also occur when a downwardly propagating waveundergoes critical refraction at the interface between the low-velocitylayer 5 and the basement 6, to generate a seismic wave propagating alongthe interface between the low velocity layer 5 and the basement 6. Thewave propagating along the interface will excite both p-mode and s-modewaves in the low velocity layer, and the invention can be applied tothese p- and s-modes.

For any parameter indicative of an aspect of the seismic data such as,for example, the pressure or a component of the particle motion (theterm “particle motion” includes particle displacement, particlevelocity, particle acceleration and higher derivatives of the particledisplacement), events of one of the p- and s-modes will generally appearmore strongly than will events of the other mode. According to preferredembodiments of the invention, therefore, two parameters indicative oftwo different aspects of the seismic data are used to locate a p-modeevent and the corresponding s-mode event. In principle, if sufficientlystrong mode conversion occurs at the lower boundary of the low-velocitylayer, corresponding pairs of p-mode and s-mode events can be located byinspection of the received seismic data. In many cases, however, theamplitude of the mode-converted events is low, and a cross-correlationor de-convolution technique is then preferred.

The static shift for a p-wave is usually small compared to the staticshift for an s-wave, since the p-wave velocity is greater than thes-wave velocity and hence the variation in travel time is smaller.Moreover, the less-consolidated sea bed sediments found in thelow-velocity layer 5 tend not to cause lateral variations in thevelocity of p-waves as significantly as they cause lateral variations inthe velocity of s-waves. In many cases, the p-wave static shift is sosmall that it can safely be assumed to be negligible.

The present invention makes it possible to estimate the differencebetween s-wave static shift and the p-wave static shift at eachreceiver. If the p-wave static is known, can be estimated, or can beassumed to be zero, then the invention makes it possible to obtain thes-wave static.

A second aspect of the present invention provides a method of seismicsurveying comprising the steps of: directing seismic energy propagatingin a first mode towards a boundary face of a layer of the seabed suchthat partial mode conversion of the seismic energy into a second modeoccurs at the boundary face; acquiring seismic data including the firstand second modes of seismic energy at one or more receivers; andprocessing the seismic data according to a method as defined above.

A third aspect of the present invention provides an apparatus forprocessing seismic data including first and second modes of seismicenergy, the apparatus comprising means for processing the seismic datato obtain the travel time difference through the layer between seismicenergy propagating in the first mode and seismic energy propagating inthe second mode.

In a preferred embodiment the apparatus comprises a programmable dataprocessor.

A fourth aspect of the invention provides a storage medium containing aprogram for the data processor of an apparatus as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described indetail by way of illustrative example with reference to the accompanyingfigures in which:

FIG. 1 is a schematic view of a marine seismic survey;

FIG. 2 is a schematic illustration of mode conversion of an up-goingp-wave at the boundary between the basement and a low-velocity layer;

FIG. 3 shows schematic examples of seismic data traces for the situationof FIG. 2;

FIG. 4 shows a cross-correlogram for the traces of FIG. 3;

FIGS. 5( a) and 5(b) show synthetic seismic data generated for the modelof Table 1;

FIGS. 6( a) and 6(b) show the deconvolution of the data of FIGS. 5( a)and (b);

FIG. 7 shows theoretical and experimental results for the pp-ps traveltime difference;

FIG. 8 is a schematic illustration of a seismic surveying arrangementhaving common-offset source and receiver geometry;

FIGS. 9( a) and 9(b) show the vertical and radial components of particlevelocity obtained using the seismic surveying arrangement of FIG. 8;

FIGS. 10( a) and 10(b) show the vertical and radial components ofparticle velocity of FIGS. 9( a) and 9(b) after preliminary processing;

FIG. 11 is a schematic illustration of mode conversion of an up-goings-wave at the boundary between the basement and a low-velocity layer;

FIG. 12( a) is a schematic illustration of mode conversion of adown-going p-wave upon reflection at the boundary between a low-velocitylayer and the basement;

FIG. 12( b) is a schematic illustration of mode conversion of adown-going s-wave upon reflection at the boundary between a low-velocitylayer and the basement;

FIG. 13( a) is a schematic illustration of mode conversion of a p-wavepropagating along the boundary between a low-velocity layer and thebasement;

FIG. 13( b) is a schematic illustration of mode conversion of an s-wavepropagating along the boundary between a low-velocity layer and thebasement; and

FIG. 14 is a schematic block diagram of an apparatus according to thepresent invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The principle of the invention will now be described with reference toan embodiment in which the pair of corresponding events are generated bypartial mode conversion. FIG. 2 illustrates the occurrence of modeconversion as an upwardly propagating p-wave passes through theinterface between the basement 6 and a low-velocity layer 5. Theup-going p-wave 7 is partially transmitted as a p-wave 7′, but alsoundergoes partial conversion to an s-wave 7″. It is assumed that theupward-going p-wave was generated by a reflection from a deep interfacebetween two layers of the earth's interior, or by refraction throughdeeper layers. Refraction will also occur at the interface between thebasement 6 and the low-velocity layer 5, and the angle of refraction forthe transmitted p-wave 7′ will be different from the angle of refractionfor the converted s-wave 7″.

The transmitted p-wave 7′ and the converted s-wave 7″ will both producea signal at the receiver array. The transmitted p-wave 7′ and theconverted s-wave 7″ are, as indicated in FIG. 2, travelling in differentdirections and hence will arrive at different receiver positions, not atthe same receiver. However, a similar converted s-wave from aneighbouring conversion point will arrive at the same receiver as thetransmitted p-wave 7′ shown in FIG. 2. It will be assumed that, for atransmitted p-wave and a converted s-wave received at a particularreceiver, the difference between the conversion point (at the base ofthe low-velocity layer) of the converted s-wave and the point in thebase of the low-velocity layer through which the p-wave was transmitteddoes not significantly affect estimates of the static shifts obtained bythe method of the present invention.

The transmitted p-wave 7 will be recorded predominantly on the verticalcomponent of seismic energy recorded at the receiver, while theconverted s-wave 7″ will be recorded predominantly on the radialcomponent of the seismic energy recorded at the receiver 4. (The radialdirection is the source-receiver direction projected onto the sea-bed,and this 5 direction will be defined to be the x-direction.) This isbecause in a practical seismic surveying arrangement waves that havepropagated up from deep in the earth's interior tend to make an angle of30° or less to the vertical for a typical structure of the earth'sinterior and a typical depth of the target reflector. The shear wave isrecorded predominantly on the radial component because the particlemotion for a shear wave is perpendicular to the direction ofpropagation, compared to the particle motion for a p-wave which is alongthe direction of propagation.

FIG. 3 is a schematic illustration of two parameters indicative ofaspects of seismic data recorded at a receiver 4 where partialmode-conversion as shown in FIG. 2 occurs. The two parameters are theradial component (x-component) of the particle velocity measured at thereceiver, and the vertical component (z-component) of the particlevelocity measured at the receiver. Since the s-wave 7″ was created as aconversion from the up-going p-wave 7, if the interface between thebasement and the low-velocity layer is approximately planar locally,then the recorded s-wave will contain a similar wavelet to the p-arrivalsignal. The s-wave arrival will not occur at the same time as the p-wavearrival but will arrive after a time delay dt (compared to the p-wavearrival) caused by the difference between the velocity of p-waves andthe velocity of s-waves in the low-velocity layer 5. This time delay dtis exactly the difference between the p-wave static shift and the s-wavestatic shift. Thus, if the p-wave static shift is known or can beestimated accurately, the s-wave static shift can be obtained simply byadding the time delay dt to the p-wave static shift.

In principle, the time delay dt can be obtained by any method ofcomparing the p-wave arrival time with the s-wave arrival time. Inpreferred embodiments of the invention, however, the time delay dt isdetermined by either deconvolving or cross-correlating a verticalcomponent and a horizontal component of the seismic energy recorded at aseismic receiver.

FIG. 4 shows the cross-correlogram obtained by cross-correlating thehorizontal and vertical components of the particle velocity shown inFIG. 3. It will be seen that there is a peak at zero time shift, whicharises because the p-wave 7′ arrival has a non-zero amplitude in thex-component of the measured particle velocity (although it occurspredominantly in the z-component of the measured particle velocity). Anys-wave energy that appears in the vertical component of the measuredparticle velocity will also cause a peak in the cross-correlogram atzero time shift.

The peak at time dt in the cross-correlogram occurs from the p-wavearrival peak in the vertical component of the measured particle velocityand the s-wave arrival peak in the x-component of the measured particlevelocity. In order to distinguish this peak accurately from the peak atzero time shift, it is desirable (although not essential) that the peaksdo not overlap. The peaks will not overlap if the wavelet in therecorded data is of sufficiently high frequency that the thickness ofthe low-velocity layer 5 is greater than two wavelet lengths. If it isnot possible to be certain that the thickness of the low-velocity layer5 is sufficiently thick to prevent the peaks overlapping, the accuracyof the determination of the time shift dt can be increased bydeconvolving the x-component and the z-component of the measuredparticle velocity. Alternatively, the recorded wavefield can becompletely decomposed into its p-wave component and its s-wave componentusing, for example, a technique such as disclosed in co-pending UKpatent application No 0001355.7 or 0003406.6, by Robertsson et al in“Wavefield separation and estimation of near surface properties usingdensely deployed 3C single sensor groups in land surface seismicrecordings”, 70th Annual Society of Exploration Geophysicists (SEG)Meeting, Calgary (2000) or by Curtis et al in “Wavefield separation andestimation of near surface properties in land seismic”, 62nd EAGEConference Glasgow, Extended Abstracts (2000). Such decompositiontechniques have the effect of removing the “leakage” peaks shown in FIG.3 from the x-component and the z-component of the measured particlevelocity. If the decomposed p-component and decomposed s-component ofthe recorded wavefield are then cross-correlated, the peak at zero timedelay should not exist while the peak at the p-s delay time dt should beenhanced compared to that shown in FIG. 4.

If the seismic waves propagate predominantly in the radial direction(rather than reflecting back into the negative -radial direction, forexample), then the cross-correlation or deconvolution of the verticaland radial components would be expected to give a positive peak at thep-s delay time dt, because the velocity of seismic energy increasesgenerally downwards across the interface between the low-velocity layer5 and the basement 6. Cross-correlation or deconvolution between thevertical and transverse components of the recorded seismic energy shoulddetect wave propagation that is not in the radial-vertical plane, forexample caused by near-surface scattering.

In a preferred embodiment, cross-correlograms obtained at a receiver formany different shots are averaged to increase the signal-to-noise ratioof the resultant cross-correlogram, known as a stackedcross-correlogram. This should increase the ratio between the amplitudeof the time-shift peak on the vertical-radial cross-correlogram ordeconvolution to the amplitude on the vertical-transversecross-correlogram or deconvolution, if the interface between thelow-velocity layer and the basement is approximately locally horizontaland the low-velocity layer is approximately azimuthally isotropic.

The invention is not limited to the use of deconvolution orcross-correlation techniques to obtain the travel time differencethrough the low velocity layer, and any suitable technique can be used.In principle, any algorithm that correlates two traces can be used. Oneexample of another suitable technique is a bicoherence time delayestimation method, as described by L. Ikelle in “Geophysics”, Vol 62,p1947 (1997), using the radial and vertical components of particlevelocity (or, as will be discussed below, the pressure and the radialcomponent of particle velocity).

FIG. 2 illustrates mode conversion occurring for an up-going p-wave. Asimilar effect occurs if the up-going wave is an s-wave, as shown inFIG. 11. When an up-going s-wave 8 passes through the boundary betweenthe basement 6 and the low-velocity layer 5, it will undergo partialtransmission as an s-wave 8′ and partial conversion to a p-wave 8″. Boththe converted p-wave 8″ and the transmitted s-wave 8′ will thenpropagate through the low-velocity layer 5 to the receivers 4. Hence,the only difference between this case and the case illustrated in FIG. 2is that the amplitude of the s-wave arrival will now be greater than theamplitude of the p-wave arrival. The time delay between a p-wave arrivaland the corresponding s-wave arrival will still be dt as in the exampleof FIGS. 2 and 3. Hence, a cross-correlation or deconvolution betweenthe vertical and horizontal components of the seismic data should againexhibit a peak at the p-s time shift (that is, at the difference betweenthe time for p-waves to travel through the low-velocity layer and thetime for s-waves to travel through the low-velocity layer 5).

The up-going shear wave in general will refract towards the normal tothe interface. However, considering the usual decrease in s-modevelocities on entering the low-velocity layer 5 from below, the s-pconverted wave will generally refract away from the normal. This shouldbe taken into account when selecting the offset range over which thedeconvolution or cross-correlation techniques are applied.

One application of the invention is in extracting the difference betweenthe p-wave static shift and the s-wave static shift from marine seismicdata recorded using a receiver disposed on the sea-bed. In this case,the p-wave static shift is usually so small that it can be ignored, sothat the p-s static shift extracted from the data by the method of thepresent invention is approximately equal to the s-wave static shift.

To illustrate the method of the invention, synthetic seismic data wasgenerated for a simple 1-D model using a reflectivity model of the typedisclosed by B. L. H. Kennett in “Seismic Wave Propagation In StratifiedMedia”, Cambridge University Press, Cambridge, England (1983). Detailsof the model used are shown in table 1, and it can be seen that themodel consists of a low-velocity layer having a thickness of 100 m,disposed between a layer of water and a basement layer. The water layeris assumed to have an infinite depth and the basement layer is alsoassumed to have an infinite depth. An explosive point source of seismicenergy deeper in the earth (representing reflections from deeper layers)was used as the source of seismic energy, and this was located 200 mbeneath the interface between the low-velocity layer and the basementlayer. A linear array of receivers was disposed on the sea-bed

TABLE 1 P-velocity S-velocity Density Thickness Layer [m/s] [m/s][kg/m³] [m] Water 1500 0 1000 Infinite Low- 1600 500 1400 100 VelocityBasement 2500 1500 1700 Infinite Source depth: 200 m below low-velocitylayer/basement interface Source type: Explosive point source, generatingp-waves only, 100 Hz Ricker wavelet Receiver position: On thewater/low-velocity layer interface, offset 0–100 m spacing 5 m

The synthetic data obtained from this model is shown in FIGS. 5( a) and(b). FIG. 5( a) shows the vertical component of particle velocity, andFIG. 5( b) shows the radial component of particle velocity.

It is assumed in the model that the source generates p-waves. The signalrecorded by the receivers contains a transmitted p-component, and ans-component produced by mode conversion occurring at the interfacebetween the basement and the low-velocity layer. The arrival of thetransmitted p-component is visible predominantly on the verticalcomponent of the recorded seismic data, as shown in FIG. 5( a). On theradial component of the data, shown in FIG. 5( b), the transmittedp-wave energy and the arrival of the p-s converted wave are visible,particularly at increasing offset.

For a typical sea-bed seismic reflection survey, most of the transmittedp-energy will be recorded on the vertical component of the receiver. Ingeneral, the velocity of seismic energy sharply decreases on entering alow-velocity layer from below, so that the ray paths of the transmittedwaves will be strongly refracted towards the vertical. In contrast, thes-wave generated by conversion at the interface between the basement andthe low-velocity layer will appear predominantly on the radial componentof the seismic energy recorded at the receiver.

It will be assumed that the interface between the basement 6 and thelow-velocity layer 5 is horizontal, so that no s_(h)-mode wave isgenerated during the p-mode to s-mode conversion at the interface. Thetangential component of the recorded data will be zero, and theconverted s wave will be a pure s_(v) wave. The theoretical displacementresponse for the low-velocity layer can then be written, following C. A.Langston in “J Geophys Res.” Vol. 84, pp 4749–47462 (1979), as:D _(V)(t)=S(t)*E _(v)(t)  (1)D _(R)(t)=S(t)*E _(R)(t)

In equations (1) S(t) is the source time function of the incidentp-wave, D_(V)(t) and D_(R)(t) are the vertical and radial components ofthe data, and E_(V)(t) and E_(R)(t) are the vertical and radial impulseresponses, respectively. By deconvolving the vertical component of therecorded data from the radial component, the difference in travel timebetween the transmitted p-wave and the converted s-wave can accuratelybe extracted from the data. The fundamental ideal underlying thisprocess is that the vertical component of the recorded data can beregarded as a good estimation of the source wavelet, because theimpulse-response of the low-velocity layer 5 for the vertical componentof the data, for an incoming planar p-wave, is close to a δ-function(Langston, supra). In other words:E _(V)(t)≅δ(t) or S(t)≅D _(V)(t)

P-wave reverberations will also be on the vertical component but theseare diminished in amplitude very much. The deconvolution result willtherefore consist of the transfer function (called a receiver functionin earthquake seismology) of the low-velocity layer layers for shearwave energy:

$\begin{matrix}{{E_{R}(\varpi)} = {\frac{{D_{R}(\varpi)}\;}{S(\varpi)} \cong \frac{D_{R}(\varpi)}{D_{v}(\varpi)}}} & (2)\end{matrix}$

The result in the time-domain can be interpreted much like areflectivity series.

Because deconvolution basically is a division in the frequency domain,it can become unstable. Therefore, the denominator is preferablyprevented from taking on too small values. This is achieved throughfilling of the spectral holes of the denominator to a fraction, c, ofits maximum, a technique suggested by S. K. Dey-Sarkar and R. A. Wigginsin “J. Geophys. Res”, Vol. 8 pp 3633–3641 (1976). Another problem thatcan arise is that noise can lead to high frequencies in the deconvolvedresult that are unphysical and not clearly present in the data. This canbe prevented by multiplying the result in the frequency domain with aGaussian function centred on zero and using the gaussian widthparameter, a, to control its width (and thus the frequency content). Thestable deconvolution in the frequency domain then is given by (Langston,supra):

$\begin{matrix}{{E_{R}(\varpi)} = {\frac{{D_{R}(\omega)}{D_{V}^{*}(\omega)}}{\Phi_{SS}(\omega)} \cdot {G(\omega)}}} & (3)\end{matrix}$where,Φ_(SS)(ω)=max{D _(V)(ω)D _(V) *,c.max[D _(V)(ω)D _(V)*]} and G(ω)=e^(−ω) ² ^(/4a) ²

Here D_(R)(ω) and D_(V)(ω) are the Fourier transforms of the radial andvertical component of the data respectively and the star denotes complexconjugate. E_(R)(ω) is the deconvolved radial earth response in thefrequency domain and can be directly transformed back to the time domainto give the desired receiver function. Φ_(SS)(ω) and G(ω) are thedescribed stabilised denominator and Gaussian ‘filter’ respectively.

Due to non-vertical incidence, there will be some p-wave energy on theradial component of the data (FIG. 5( b)). This will also be present inthe deconvolved result where it should be exactly at zero time. Becauseof the stabilisation of the deconvolution the width of the receiverfunction pulses becomes finite and therefore part of this will bewrapped around the end of the receiver function trace (FIG. 6( a)). In apreferred embodiment, this is prevented by padding zeros—at the length nof the trace—in front of the radial component and by padding the sameamount of zeros behind the vertical component. The deconvolved trace(length 2n) then will have its zero time at sample n+1 and no arrivalwill be wrapped around (FIG. 6( b)).

Ideally the receiver function thus can be interpreted as the shear waveresponse of the low-velocity layer to an incident plane P-wave frombelow. The amplitudes of the receiver function therefore containinformation on the low-velocity layer medium properties (i.e. velocityand density) and the low-velocity layer/basement velocity contrast.However, stabilising the deconvolution through filling of the spectralholes changes the amplitude of the receiver function. Following asuggestion made by C. J. Ammon in “Bull. Seismol. Soc. Am.” Vol 81 pp2504–2510 (1991), the true amplitudes of the receiver function can berecovered. The effect of stabilising the deconvolution can be estimatedby deconvolving the vertical component from itself (using the samevalues for parameters a and c), knowing the maximum amplitude of thisdeconvolved trace should consist of a single peak at zero time withamplitude one. Therefore, by dividing the receiver function by themaximum of this auto-deconvolution the true amplitudes are recovered.

FIG. 7 is an illustration of the theoretical travel time differencebetween the transmitted p-wave and the converted p-wave calculated fromthe parameters of the model of Table 1, compared against the measuredtime difference as taken from the synthetic receiver traces of FIGS. 5(a) and (b). In FIG. 7, the theoretical travel time difference is shownas a broken line, and the measured time difference extracted from thesynthetic data of FIGS. 5( a) and (b) is shown as a solid line. It willbe noted that the solid line has a 0.2 ms stair step, and this arisesbecause a sampling time interval of 0.2 ms was used in the generation ofthe synthetic seismic data. It will be seen that the time delay obtainedby deconvolving the synthetic data of 5(a) and (b) agrees well with thetheoretical travel time difference calculated from the model parameters.

In the model of table 1, the data is generated by a source locatedwithin the earth's interior, beneath the lower boundary of thelow-velocity layer 5. However, the techniques described above withrelation to the model of table 1 can be applied in exactly the samemanner to data obtained using seismic energy that was emitted from asurface source and that has been refracted or reflected from layersdeeper within the earth's interior so that it propagates upward throughthe part of the model of the earth's interior used in this method.

Using the vertical and the radial components of the particle velocity asthe two parameters indicative of the seismic data, as described above,has the advantage that the p-mode events and s-mode events are separatednaturally, with p-mode events appearing predominantly on the verticalcomponent and shear wave events appearing predominantly on the radialcomponent. However, the invention is not limited to the use of thevertical and the radial components of the particle velocity as the twoparameters indicative of the seismic data.

In the deconvolution of the synthetic data of FIGS. 5( a) and 5(b), thedeconvolution was performed for all offsets, using the full time lengthof the synthetic traces. This was possible because of the extremesimplicity of the earth model for which the synthetic data wascalculated and the simple acquisition geometry (a transmissionsource-receiver setting, with the source being located in the basementregion of the model). This simple straightforward approach cannotgenerally be used in practice, however. In a practical survey seismicdata are acquired using more complicated acquisition geometries, anddeconvolving the data for all offsets over the full time length of thetraces would generally require considerable processing power. It istherefore preferable to perform the deconvolution or cross-correlationfor a selected range of time and/or offset to reduce the processingrequired. It is also preferable to select the range of time and/oroffset so as to reduce noise in the results of the deconvolution orcross-correlation.

As mentioned in the above explanation of the theoretical background ofthe deconvolution method, the deconvolved result is essentially theshear-wave impulse response of the low-velocity layer when acompressional wave is incident on it from below. Deconvolution andcross-correlation can therefore be seen as ways of determining a filterrepresenting the low-velocity layer, with in this case a specialinterest in the phase-delay part of the filter. Put in this way, it willbe clear that no down-going energy, such as water-layer reverberationsand the direct wave from the source to the receiver, should be presentin the selected data window. Moreover, no waves converted to shearenergy in the basement should be present in the selected data (unlesss-to-p conversion at the interface of the low-velocity layer is beingused to find the difference between the p-static shift and the s-staticshift, as will be discussed below with reference to FIG. 11). Thesetypes of seismic energy have not been filtered by the low-velocity layerin the way considered here and can therefore be regarded as noise forthis method. The data used for the deconvolution/cross-correlationprocess is preferably selected to remove this energy as much aspossible.

The time length of a window used to select data for thedeconvolution/cross-correlation process is preferably short, to minimizethe processing required. Although several consecutive up-going reflectedp-wave events could in theory be filtered (i.e. partially converted toshear waves) by the low-velocity layer in exactly the same manner, ithas long been established in the field of spectral estimation thattaking longer data windows, although the filter relationship betweenboth components is the same, does not reduce the variance of the result.In addition longer time-length windows potentially contain more unwantedarrivals, leading to additional noise in the results of thecross-correlation or deconvolution process. The time window forselecting the radial component of the data should be chosen to cover theshear wave arrival that has a time delay corresponding to the maximumexpected time delay.

In selecting a part of the data for deconvolution/cross-correlation itis also preferable to select data for offset. Zero-offset data for aplane layered medium will not contain any shear-wave energy arising froms-wave to p-wave conversion at the boundary of the low-velocity layer,since no conversion will take place for vertically incidentcompressional waves. On the other hand, data with a long offset (thatis, data where the offset is substantially greater than the reflectordepth) will not satisfy the surface-consistent statics assumption andthe static corrections will become dynamic corrections. It is thereforepreferable to exclude zero-offset data and long-offset data from thedata used for deconvolution/cross-correlation.

For a survey site where there is pre-existing knowledge of the structureof the low-velocity layer and the basement, it may be possible tocalculate an offset range in which the maximum p-s conversion isexpected to occur. If so, the offset range selected fordeconvolution/cross-correlation should include the offset range wheremaximum p-s conversion is expected to occur.

In another embodiment, (scaled) pressure recordings are used instead ofthe vertical component of particle velocity. It has been observed by X.Li et al, in “Lomond Data Analysis: Geophone Coupling and Converted-WaveImaging, Research Report Edinburgh Anisotropy Project” (Applied SeismicAnisotropy), Vol. 7, Converted waves II: Case examples, pp. 185–212(1999/2000), that leakage of small amounts (<5%) of shear wave energyfrom the radial component of particle velocity to the vertical componentof particle velocity can occur owing to unwanted cross-coupling betweenthe two components of particle velocity using seabed acquisition cables.This unwanted cross-coupling can be accompanied by a small phase shiftof the order of 6 ms, and therefore will appear in the deconvolution andcross-correlation results as a static event with a ‘traveltimedifference’ of order 6 ms. No such coupling effects exist betweenpressure and the radial component of particle velocity and thereforereceiver functions calculated by deconvolving or cross-correlatingpressure from the radial component of particle velocity do not sufferthis phase shift. However because of the scalar nature of a pressurerecording, the zero-time or projection part in the deconvolution orcross-correlation results will be more significant.

In the embodiments described above the time shift dt has been determinedby deconvolving or cross-correlating parameters indicative of themeasured seismic data. Such techniques are advantageous where themode-converted events have a low amplitude. If the contrasts inproperties between the low-velocity layer 5 and the basement 6 are morethan sufficiently strong for the method of the invention to work, thep-s converted events can have such high amplitudes that it may not benecessary to perform deconvolution or cross-correlation (i.e. toexplicitly calculate receiver functions) from which the traveltimedifferences may be estimated. Instead, the p-s converted phase may beobserved directly in the seismic data.

FIG. 8 illustrates a seismic surveying arrangement in which data isacquired by source-receiver pairs which all have the same offset—theoffset is equal in both magnitude and sign for every source-receiverpair. This is known as Common (signed) Offset Profile acquisitiongeometry. The sources emit p-mode seismic energy, which is reflected bythe reflector 3 back to the receivers 4. Mode conversion occurs when thereflected p-wave passes through upwards through the interface betweenthe basement 6 and the low-velocity layer 5, so that the seismic energyreceived at the receivers contains both p-modes and s-modes. The offsetof the source receiver pairs is chosen to correspond to the range inwhich the maximum p-s conversion is expected to occur (the offset is notzero-offset and is not too great).

FIGS. 9( a) and 9(b) show respectively the vertical component and theradial component of the particle velocity measured by the receivers 4 inthe surveying arrangement of FIG. 8. It will be seen that the reflectedp-event appears predominantly in the vertical component of the particlevelocity, whereas the converted s-event appears predominantly in theradial component of the particle velocity. Some slight leakage occurs,so there are weak events corresponding to the reflected p-event in theradial component of the particle velocity and weak events correspondingto the converted s-event in the vertical component.

The p-s converted events in the radial component of the particlevelocity have sufficient amplitude to be directly visible. Thus, byinspecting the radial component of the particle velocity at sample timesshortly after (0–100 ms after) a strong p-event has been recorded in thevertical component, the corresponding p-s converted event may beobserved directly. To extract the travel time difference between thep-wave and the corresponding converted s-wave, the p-event can betracked across the traces produced by different receivers. Its arrivaltime can be used as a zero-time reference to define the start of atime-window within which the corresponding s-event is expected to appearin the radial component. The length of the time window corresponds tothe maximum expected shear static. Once the corresponding s-event hasbeen located within the time window, the travel time difference betweenthe p-event and the s-event can then be read directly from the data.Alternatively, the s-event can again be tracked across the traces.

The p-event is not “flat” in the traces shown in FIGS. 9( a) and 9(b),since it does not occur at the same time in every trace. FIGS. 10( a)and 10(b) show the data of FIGS. 9( a) and 9(b) after they haveundergone preliminary processing to make the p-event “flat”.

The embodiments of the invention described above relate to modeconversion occurring when an upwardly propagating wavefield undergoespartial mode conversion upon transmission through the interface betweenthe basement 6 and the low-velocity layer 5. It is, however, possiblefor mode conversion to occur when a downwardly propagating waveundergoes reflection at the interface between the low-velocity layer 5and the basement 6. This is illustrated schematically in FIGS. 12( a)and 12(b).

In FIG. 12( a), a p-wave 7 propagates downwardly through thelow-velocity layer 5 and is reflected at the interface between thelow-velocity layer 5 and the basement 6. Partial mode conversion occursupon reflection so that, in addition to the up-going reflected p-wave7′, there also exists an up-going mode-converted s-wave 7″. Thetechniques described above for determining the difference between thep-wave static shift and the s-wave static shift can be applied equallyto the survey arrangement shown in FIG. 12( a). The method can beapplied in exactly the same way as it is applied to the upgoingtransmitted (and partially converted) wavefield except that datarecorded at shorter offsets have to be used when the reflections of adowngoing source wavefield are considered. This is because the wavefieldreflected by a deep reflector in the model used in the transmissivearrangements will be propagating closer to the vertical at a givenoffset, compared to waves reflected at interface between thelow-velocity layer 5 and the basement arriving at the same offset.

FIG. 12( b) generally corresponds to FIG. 12( a), but illustratespartial mode conversion occurring on reflection of a downgoings-wavefield 8 to give an upgoing s-wavefield 8′ and a upgoing modeconverted p-wavefield 8″.

FIGS. 13( a) and 13(b) illustrate further embodiments of the invention.In these embodiments mode-conversion is induced by a wave field thatpropagates downwardly through the low velocity 5 and undergoes criticalrefraction at the interface between the low velocity layer 5 and thebasement 6.

FIG. 13( a) shows a p-wave 9 propagating downwards through the lowvelocity layer 5. The angle θ_(i) denotes the incident angle—that is,the angle between the direction of propagation of the wave and thenormal to the interface (or, in the case of a non-planar interface, theinstantaneous normal to the interface). When the p-wave 9 is incident onthe interface between the low velocity layer 5 and the basement, onewould ordinarily expect that the p-wave 9 would be partially transmittedinto the basement 6, as shown by the broken line 9′ and partiallyreflected at the interface. The angle θ_(r) between the transmitted wave9′ and the normal to the interface is related to the incident angleθ_(i) by Snell's Law. The velocity of seismic energy in the basement 6will generally be greater than the velocity of seismic energy in the lowvelocity layer 5, however, so that if the angle of incidence exceeds acertain critical angle then Snell's law will predict a value for sinθ_(r) that is greater than 1. When this happens a refracted wave 9″ willpropagate along the interface between the low velocity layer 5 and thebasement 6. This phenomenon is known as “critical refraction”, and issimilar to the phenomenon of total internal reflection in optics. Thecritically refracted wave can excite upwardly propagating waves, knownas “head waves”, in the low velocity layer, and these are recorded atthe receivers 4. Such head waves are found in seismic data whenever thesource-receiver offset is sufficiently great that the incident angleθ_(i) on the relevant interface exceeds the critical angle for the onsetof critical refraction. In the case of the interface between the lowvelocity layer 5 and the basement 6 the critical offset is low, and headwaves are recorded for most receiver locations and for most shot points.

As an example, consider a low velocity layer 5 consisting of a materialwith p-wave propagation velocity α₁ and an s-wave propagation velocityβ₁ overlying a basement 6 with a p-wave propagation velocity α₂ and ans-wave propagation velocity β₂, where the following relations aresatisfied:α₁<α₂β₁<β₂; andβ₂<α₁

FIG. 13( a) illustrates the case where the incident angle θ_(i)satisfies the relationship sin θ_(i)=α₁/α₂. This incident angle is afirst critical angle, and will be referred to as θ_(cαα.)

When the incident angle of the down-going p-wave 9 satisfiesθ_(i)=θ_(cαα) sin θ_(r) takes the value of 1, meaning that θ_(r) is 90°.The transmitted wave 9′ propagating into the basement 6 will thereforenot exist, and instead there will be a critically refracted p-wave 9″that propagates along the interface between the low velocity layer 5 andthe basement 6. As this refracted p-wave propagates along the interfaceit will excite p-waves 10 within the low velocity layer 5, and thesep-waves 10 will give rise to s-waves 11 as a result of mode conversionat the interface between the low velocity layer 5 and the basement 6.Two pairs of the p-wave 10 and the mode-converted s-wave 11 are shown inFIG. 13( a), although the excitation of the p- and s-waves 10, 11 willoccur at all points along the path of the refracted wave 9″. The excitedp-wave 10 will propagate at an angle of θ_(cαα) to the normal to theinterface, whereas the mode-converted s-wave 11 will propagate at anangle θ_(cβα) to the normal, where sin (θ_(cβα))=β₁/α₂. The criticallyrefracted p-wave occurs for all incident angles equal to or greater thanθ_(cαα).

FIG. 13( b) corresponds generally to FIG. 13( a), but illustrates thecritical refraction of an s-wave 12 propagating downwardly through thelow velocity layer 5. If the angle of incidence θ_(i) is equal to orgreater than a second critical angle θ_(cβα), then the downgoing s-wave12 will partially convert to a critically refracted p-wave 12″ whichpropagates along the interface between the low velocity layer 5 and thebasement 6. The critically refracted p-wave again excites p-waves 13 andmode-converted s waves 14 that propagate upwardly through the lowvelocity layer 5.

In the cases illustrated in 13(a) and 13(b), the critically refractedp-wave 9″, 12″ emits both p-wave energy and s-wave energy upwards intothe low velocity layer 5. Detection of both the p-wave and s-wave energyat the receivers 4 again yields the difference between the static shiftof p-waves and the static shift of s-waves. The s-wave 11 is formed bymode-conversion at the interface between the low velocity layer 5 andthe basement 6, so that the travel time difference through the lowvelocity layer between a p-wave 10 excited at one point on the interfaceand the corresponding mode-converted s-wave 11 will be equal to thedifference between the p-mode static shift and the s-mode static shift.Thus, the time difference between the arrival time of the p-wave 10 (thep-arrival) and the arrival time of the s-wave 11 (the s-wave arrival) atthe receivers 4 is equal to the difference between the static shift forthe p-wave and the static shift for the s-wave. Thus, by detecting thetime delay between a p-arrival and the corresponding s-arrival, it ispossible to determine the difference between the p- and s-static shifts.The time delay can be determined using any of the methods describedabove, for example, by deconvolution or cross-correlation of thevertical and radial components of the measured particle velocity, or bydeconvolution or cross-correlation of the measured pressure and theradial component of the particle velocity. It should be noted that thereexists a third critical angle, θ_(cββ), where sin θ_(cββ)=β1/β₂. When adown-going s-wave is incident on the interface between the low velocitylayer 5 and the basement 6 at an angle equal or greater to the thirdcritical angle θ_(cββ), a critically refracted s-wave will be generated,which propagates along the interface between the low velocity layer 5and the basement 6. However, this critically refracted s-wave will notproduce an upwardly propagating p-wave in the low velocity layer 5.

In the examples of FIGS. 13( a) and 13(b) it has been assumed that boththe p-wave velocity and the s-wave velocity increase downwards acrossthe interface between the low velocity layer and the basement. It issometimes found that not all velocities increase downwards across theinterface, and this leads to situations that are more complicated thanthose shown in FIG. 13( a) and 13(b). However, whenever criticalrefraction occurs at the interface between the low velocity layer 5 andthe basement 6, leading to excitation of both p-waves and mode-converteds-waves into the low velocity layer 5, it is possible to apply themethod of the present invention. The seismic energy acquired at thereceivers 4 can be analysed by any method described above to extract thetravel time difference for the emitted p-wave energy and the s-waveenergy, and thus obtain the difference between the p-wave static shiftand the s-wave static shift.

In the above examples of the invention, the travel time difference forp-modes and s-modes through the low-velocity layer is obtained from twocomponents of the particle velocity, or from a component of the particlevelocity and the pressure. In alternative embodiments of the invention,the measured particle displacement is used rather than the particlevelocity. Thus, in an alternative embodiment the travel time differencefor p-modes and s-modes through the low-velocity layer is obtained fromtwo components of the particle displacement, for example bycross-correlating or deconvolving the vertical and radial components ofthe particle displacement. In a further embodiment, the travel timedifference for p-modes and s-modes through the low-velocity layer isobtained from a component of the particle displacement and the pressure,for example by cross-correlating or deconvolving a component of theparticle displacement and the pressure. These embodiments correspondgenerally to the embodiments described above with regard to using theparticle velocity to obtaining the travel time difference, and will notbe described further.

In further embodiments of the invention the particle acceleration isused rather than the particle velocity. Thus, in these embodiments thetravel time difference for p-modes and s-modes through the low-velocitylayer is obtained from two components of the particle acceleration, orfrom a component of the particle acceleration and the pressure. This canbe done by, for example, cross-correlating or deconvolving the verticaland radial components of the particle acceleration or bycross-correlating or deconvolving a component of the particleacceleration and the pressure. These embodiments correspond generally tothe embodiments described above with regard to using the particlevelocity to obtaining the travel time difference, and will not bedescribed further. In principle, higher derivatives of the particledisplacement could be used, so that the travel time difference forp-modes and s-modes through the low-velocity layer could be obtainedfrom two components of a higher derivative of the particle displacement,or from a component of a higher derivative of the particle displacementand the pressure.

In the embodiments described above the receiver(s) 4 has/have beendisposed on the sea-bed. In principle, however, the invention is notlimited to this, and the receiver(s) could be located anywhere in thepath of seismic energy after partial mode conversion has occurred. Thus,in the embodiments described above the receiver(s) could in principle belocated anywhere above the interface between the low-velocity layer andthe basement. For example, the receiver(s) could be buried within thesea-bed in a 4-D time-lapse seismic survey. It should be noted howeverthat additional processing will be required to determine the differencebetween the static shifts of the two modes if the receiver is notlocated on the sea-bed.

In the above description of embodiments of the invention the x-directionhas been defined to be the radial direction, namely the projection ofthe source-receiver direction onto the sea-bed. A seismic receiver willrecord the components of particle velocity or particle displacement intwo orthogonal horizontal directions, and these directions may bereferred to as the receiver's x- and y-axes. It should be noted that thereceiver may not be deployed with its x-axis aligned with the radialdirection, so that an additional projection or rotation of the “raw”horizontal components measured by the receiver may be required in orderto calculate the radial and transverse components from the receiver'soutput. The raw x- and z- components measured at the receiver can inprinciple be used to estimate the travel time difference for p-modes ands-modes through the low-velocity layer, by any of the methods describedabove, and this would give acceptable results if the angle between thereceiver's x-axis and the radial direction is not too large.

A processing method of the present invention may be applied to theprocessing of pre-existing seismic data. The invention may also beincorporated into a method of seismic surveying in which the acquiredseismic data includes mode-converted events arising from partial modeconversion at the interface between a low velocity layer and thebasement. Such seismic data can be obtained by directing seismic energytowards the interface such that partial mode conversion occurs whenseismic energy is transmitted through, or reflected by, the interface.

As noted above, the present invention can be applied to upwardlypropagating s-waves that undergo partial mode conversion to p-waves atthe interface between the basement and the low velocity layer, as wellas to upwardly propagating p-waves that undergo partial mode conversionto s-waves at the interface. Both these cases can be used to find thetravel time difference for s-waves and p-waves through the low velocitylayer. In principle, the travel time difference between p-waves ands-waves through the low velocity layer would depend on the slowness ofthe incident wave, even for an ideal situation in which the seabed andthe interface between the low velocity layer and the basement are bothflat. In practice, however, for the range of source-receiver offsetsthat are used in typical seismic surveys, and for typical materials ofthe low velocity layer, it is found that the dependence of the traveltime difference on the slowness of the incident wave is very weak, andthat the travel time difference is effectively constant regardless ofthe slowness of the incident wave. It has been found that a typicalvariation is only a few milliseconds across the complete receiver range.

It is therefore safe to assume that the travel time difference throughthe low velocity layer is independent of the slowness of the incidentwave. This assumption is equivalent to the surface-consistent staticassumption, which is generally considered to be valid in most staticproblems. Making this assumption means that the wave field recorded atthe receivers 4 does not need to be filtered or separated even though itis a mixture of waves arriving with different slownesses from differentreflectors within the earth, and also is a mixture of both p-waves ands-waves,. Furthermore, it is not necessary to extract single events fromthe data in order to apply the method of the invention. This means thatrelatively long time windows (of the order of seconds) can be used inthe deconvolution or cross-correlation process. Furthermore, traces canbe stacked in the common receiver domain to improve the signal-to-noiseratio of the data.

It has been noted above that the receiver may not be deployed with itsx-axis aligned with the radial direction, in which case the “raw”horizontal components measured by the receiver may need to be projectedor rotated in order to calculated the radial and transverse componentsfrom the receiver's output. It should be noted that this procedure isnot necessary where the seismic forces are arranged in a shotline, andare actuated when the shot line is disposed directly over, parallel to,the receiver line. In this case, the x-component of the raw receiverdata can be regarded as the radial component and the y component of theraw receiver data can be considered to be the trasverse component. Itshould be noted, however, that the sign of the data changes for eventson the radial component of the seismic data when the offset changes fromnegative to positive (the one-dimensional earth assumption). This mustbe taken into account when stacking deconvolution or cross correlationresults from a common receiver gather that includes both positive andnegative offsets, since neglecting this can lead to attenuation andpossibly even complete loss of the signal. It is necessary to reversethe sign of data acquired for one offset before stacking the data

The invention has been described above with reference tocross-correlation, deconvolution and bicoherence as examples of methodsfor correlating two seismic data traces. The invention is not limited tothese methods, however, and any suitable technique or algorithm forcorrelating two traces may be used.

The invention may also be applied to p- and s-wave “reverberationevents”. In such events partial mode conversion, for example upontransmission through or reflection at the interface between the nearsurface 5 and the basement 6. In a reverberation event, however, the p-and s-waves do not pass directly from the point at which themode-conversion occurs to the receiver. In a reverberation event the p-and s-waves undergo one or more reflection at a boundary of thenear-surface and so make multiple passes through the near-surface 5before being incident on the receiver.

Seismic data may also contains events that arise from partial modeconversion that occurs not at a boundary of the near-surface but withinthe near-surface. This may arise owing to, for example, layering effectsin the near-surface.

In the embodiments described above one of the p- and s- wave events hasbeen generated by partial mode conversion. However, as noted above, theinvention is not limited to this and may be applied to any pair ofcorresponding p- and s-events for which differences in amplitude and/orwaveform of the two events arise primarily from the near-surface 5. Forexample, in a seismic survey that uses a source that emits both p- ands-waves, the acquired data will contain an event arising from reflectionof p-waves at a particular point on the lower boundary of thenear-surface 5 and will also contain a corresponding event arising fromreflection of s-waves at that point on the lower boundary of thenear-surface 5. The invention may be applied to these events. (It shouldbe noted that, since a water column will not support s-wave propagation,a source emitting both s- and p-modes would need to be disposed on theseabed, on the earth's surface or in a borehole.

FIG. 14 is a schematic block diagram of a programmable apparatus 15according to the present invention. The apparatus comprises aprogrammable data processor 16 with a program memory 17, for instance inthe form of a read only memory ROM, storing a program for controllingthe data processor 16 to perform any of the processing methods describedabove. The apparatus further comprises non-volatile read/write memory 18for storing, for example, any data which must be retained in the absenceof power supply. A “working” or “scratchpad” memory for the dataprocessor is provided by a random access memory (RAM) 19. An inputinterface 20 is provided, for instance for receiving commands and data.An output interface 21 is provided, for instance for displayinginformation relating to the progress and result of the method. Seismicdata for processing may be supplied via the input interface 20, or mayalternatively be retrieved from a machine-readable data store 22.

The program for operating the system and for performing the methoddescribed hereinbefore is stored in the program memory 17, which may beembodied as a semiconductor memory, for instance of the well-known ROMtype. However, the program may be stored in any other suitable storagemedium, such as magnetic data carrier 17 a (such as a “floppy disc”) orCD-ROM 17 b.

1. A method of processing first and second seismic data sets includingfirst and second modes of seismic energy, respectively, the methodcomprising: combining the first and second seismic data sets to form athird seismic data set; and determining a travel time difference througha layer between seismic energy propagating in the first mode and seismicenergy propagating in the second mode based on the third seismic dataset.
 2. A method as claimed in claim 1 wherein the second mode has beengenerated by partial mode conversion of the first mode at a boundaryface of a layer of a seabed.
 3. A method as claimed in claim 2 whereinthe boundary face is a lower boundary face of the layer.
 4. A method asclaimed in claim 2 wherein the layer is a surface layer or anear-surface layer.
 5. A method as claimed in claim 1 wherein combiningthe first and second seismic data sets comprises cross-correlating thefirst and second seismic data sets with one another to form the thirdseismic data set.
 6. A meted as claimed in claim 1 wherein combining thefirst and second seismic data sets comprises deconvoling the first andsecond seismic data sets with one another to form the third seismic dataset.
 7. A method as claimed in claim 1 wherein combining the first andsecond seismic data sets comprises processing the first and secondseismic data sets using a bicoherence time delay method thereby toobtain the travel time difference through the layer between seismicenergy propagating in the first mode and seismic energy propagating inthe second mode.
 8. A method as claimed in claim 1 further comprisingdetermining a travel time through the layer of the first mode from thefirst seismic data set and determining a travel time through the layerof the second mode from the second seismic data set.
 9. A method asclaimed in claim 1, wherein the first seismic data set comprises a firstcomponent of a particle velocity and the second seismic data setcomprises a second component of the particle velacity, the first andsecond components of particle velocity not being parallel to oneanother.
 10. A method as claimed in claim 9 wherein the first componentof the particle velocity is substantially a vertical component and thesecond component of the particle velocity is substantially a horizontalcomponent.
 11. A method as claimed in claim 1, wherein the first seismicdata set comprises a first component of a particle velocity and thesecond seismic data set comprises a pressure.
 12. A method as claimed inclaim 1, wherein the first seismic data set comprises a first componentof a particle dispiacement and the second seismic data set comprises asecond component of the particle displacement, the first and secondcomponents of the particle displacement not being parallel to oneanother.
 13. A method as claimed in claim 12 wherein the first componentof the particle displacement is substantially a vertical component andthe second component of the particle displacement is substantially ahorizontal component.
 14. A method as claimed in claim 2, wherein thefirst mode is a p-wave mode and the second mode generated by partialmode conversion is an s-wave mode.
 15. A method as claimed in claim 2,wherein the first mode is an s-wave mode and the second mode generatedby partial mode conversion is a p-wave mode.
 16. A method as claimed inclaim 1 further comprising forming the first and second seismic datasets by decomposing received seismic data into a p-component and ans-component.
 17. A method as claimed in claim 16 wherein combining thefirst and second seismic data sets comprises cross-correlating thep-component and the s-component of the seismic energy with one anotherto form the third seismic data set.
 18. A method of seismic surveyingcomprising the steps of: directing seismic energy propagating in a firstmode towards a boundary face of a layer of the seabed such that partialmode conversion of the seismic energy into a second mode occurs at theboundary face; acquiring first and second seismic data sets includingthe first and second modes of the seismic energy, respectively, at oneor more receivers; combining the first and second seismic data sets toform a third seismic data set; and determining a travel time differencethrough the layer between seismic energy propagating in the first modeand seismic energy propagating in the second mode based on the thirdseismic data set.
 19. A method of seismic surveying as claimed in claim18 wherein the step of acquiring first and second seismic data setscomprises acquiring first and second seismic data sets at one or morereceivers disposed above the layer.
 20. A method as claimed in claim 18wherein the step of acquiring first and second seismic data setscomprises acquiring first and second seismic data sets at one or morereceivers disposed on the seabed.
 21. A method of seismic surveying asclaimed in claim 18, and comprising directing seismic energy downwardlythrough the layer whereby partial mode conversion occurs upon reflectionat the boundary face of seismic energy of the first mode propagatingdownwardly through the layer.
 22. A method of seismic surveying asclaimed in claim 18, and comprising directing seismic energy upwardlytowards the layer whereby partial mode conversion occurs upontransmission through the boundary face of upwardly propagating seismicenergy of the first mode.
 23. A method of seismic surveying as claimedin claim 18, and comprising directing seismic energy towards theboundary face of the layer at such an angle as to cause seismic energyin the first mode to propagate along the boundary face.
 24. A method ofseismic surveying as claimed in claim 23 wherein the first mode is ap-wave made and the angle θ_(i) between the direction of propagation ofthe incident seismic energy and the normal to the boundary facesatisfies sinθ_(i)≧α_(i)/α₂ where α_(l) is the velocity of propagationof p-waves in the layer of the seabed and α₂ is the velocity ofpropagation of p-waves in an underlying layer.
 25. A method of seismicsurveying as claimed in claim 23 wherein the first mode is a p-wave modeand the angle θ_(i) between the direction of propagation of the incidentseismic energy and the normal to the boundary face satisfiessinθ_(i)≧β₁/α₂ where ⊕₁ if is the velocity of propagation of s-waves inthe layer of the seabed and α₂ is the velocity of propagation of p-wavesin an underlying layer.
 26. An apparatus for processing first and secondseismic data sets including first and second modes of seismic energy,respectively, the second mode having been generated by partial modeconversion of the first mode at a boundary face of a layer of theseabed, the apparatus comprising: means for combining the first andsecond seismic data sets to form a third seismic data set; and means fordetermining atravel time difference through the layer between seismicenergy propagating in the first made and seismic energy propagating inthe second mode based on the third seismic data set.
 27. An apparatus asclaimed in claim 26 wherein the means for combining are adapted tocross-correlate the first and second seismic data sets with one another.28. An apparatus as claimed in claim 26 wherein the means for combiningare adapted to deconvolve the first and second seismic data sets withone another.
 29. An apparatus as claimed in claim 26 wherein the meansfor combining are adapted to process the first and second seismic datasets using a bicoherence time delay method.
 30. An apparatus as claimedin claim 26 comprising means for determining a travel time through thelayer of the first mode from the first seismic data set and means fordetermining a travel time through the layer of the second mode from thesecond seismic data set.
 31. An apparatus as claimed in claim 26 andcomprising a programmable data processor.
 32. An apparatus as claimed inclaim 31, further comprising a storage medium containing a program forthe programmable data processor.